Regulatory Sites in Hormonal Controls

Although movement (active or passive) is required, it is uncertain how far the substance must move to qualify as a hormone. Some influences such as wound hormone (Audus 1959) and others (Lang 1966, Steeves and Sussex 1972) span only a few cells. Would it be minimally sufficient for the hormone to move only from one cell to the next Or, like the neurohumors of animal systems, might it be sufficient for the hormone to move only across a membrane  

As another example, light may inhibit corn mesocotyl growth by limiting the supply of auxin transported from the coleoptile (Vanderhoef and Briggs 

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